The present invention relates to a method of producing an alkaline storage battery equipped with an electrode group having alternately stacked positive electrode and negative electrode via separators, and particularly to a method of producing an alkaline storage battery, wherein the core plates of the electrodes disposed at the outermost sides of the electrode group are exposed and the electrode group is contained in a metal-made casing such that the exposed core plates are in contact with the metal-made outer casing. Further, the present invention relates to a hydrogen absorbing alloy electrode comprising an electrically conductive core plate having attached thereto mixture containing at least a hydrogen absorbing alloy powder and a binder and to the production method thereof, and also to an alkaline storage battery equipped with an electrode group formed laminating negative electrode each coated with a hydrogen absorbing alloy powder and positive electrodes via separators in a metal-made outer casing.
Recently, with the increase of small-sized potable instruments, the demand of secondary cells (storage batteries) capable of charging and discharging has been increased, and particularly, with small sizing and thinning of instruments, and increasing of necessity to use a space efficiently, the demand of a nickel-metal hydride battery obtained a large capacity battery has been rapidly increased. In the nickel-metal hydride battery of this kind, as shown in FIG. 3, electrode group 50 a formed by laminating a positive electrode 51 using nickel hydroxide as the active material and a negative electrode 52 using a hydrogen absorbing alloy as the active material via a separator 53 are contained together with an alkaline electrolyte in a metal-made outer casing (battery casing) 55, and the metal-made outer casing 55 is tightly sealed to provide a nickel-metal hydride battery 50. The negative electrode 52 is formed by coating an active material slurry made of a hydrogen absorbing alloy on both surfaces of a core plate 52a holding an active material.
The nickel-metal hydride battery 50 of the above-described structure has a fault that an oxygen gas generated when the positive electrode 5 is fully charged permeates through the separator 53 and diffuses in the negative electrode 52 to greatly deteriorates the negative electrode 52. Particularly, the deterioration of the active material layer 52b of the negative electrode 52, which does not face the positive electrode 51, that is the active material layer 52b of the negative electrode 52 disposed at the outermost side of the electrode group 50a facing the metal-made outer casing 55 becomes severe. This is because the active material layer 52b of the negative electrode 52 disposed at the outermost side of the electrode group 50a does not face the positive electrode 51, at charging, the progress of the charging reaction becomes later than the negative electrode 52 disposed at an intermediate portion. Thereby, when the positive electrode 51 is fully charged and an oxygen gas generates, the negative electrode 52 disposed at the outermost side of the electrode group 50a is attacked by the oxygen gas in a state the occluded amount of hydrogen is less than the negative electrode 52 disposed in an intermediate, whereby the deterioration becomes severe, and the cycle characteristics of the battery is lowered.
Also, because the active material layer 52b of the negative electrode 52 at the outermost side, which does not face the positive electrode 51, becomes a region of low electrochemical activity, which does not efficiently carry out the absorbing and releasing reaction of hydrogen at charge and discharge, the active material layer 52b becomes an active material layer which is not substantially effectively utilized. The formation of the region, wherein the active material is not effectively utilized, in the inside of the battery casing 55 is the fault of lower the volume energy density, which is very important for the battery. Particularly, because the nickel-metal hydride battery is designed such that the electrode capacity of the negative electrode is larger than that of the positive electrode for employing a sealed structure by absorbing an oxygen gas in the negative electrode, when a portion, which is not effectively utilized, is formed, the volume energy density is considerably lowered.
Thus, as shown in FIG. 4, a nickel-metal hydride battery 60 is proposed in Japanese Patent Laid-Open No. 255834/1998, wherein an electrode group 60a having stacked a positive electrodes 61 and a negative electrodes 61 via a separator 63 is placed in a battery casing 65, and a core plate exposed surface 62b is formed in such manner that an active material is disposed at the negative electrode 62 of the outermost side of the electrode group 60a, opposing to the positive-electrode 61, while an active material does not exist at the side, which does not face the positive electrode 61 and opposes to the battery casing 65. By employing such a construction, the amount of the active material disposed at the negative electrode 62 of the outermost side of the electrode group 60a become a half of the amount of the active material at the negative electrode of an intermediate disposed between positive electrodes 62, and the electrode capacity thereof becomes smaller than that of the negative electrode at the intermediate.
Because the side, which is disposed at the outermost side of the electrode group and faces the battery casing, does not face the positive electrode, the side becomes a region of low electrochemical activity and does not efficiently carry out the absorbing and releasing reaction of hydrogen, but because in the nickel-metal hydride battery 60, wherein the core plate exposure surface 62b is formed by removing the active material at the side disposed at the outermost side of the electrode group as described above, the active material can be dividedly added to each negative electrode 62 at an intermediate disposed between the positive electrodes 61, the formation of the portion which is not effectively utilized can be prevented, and lowering of the volume energy density can be prevented.
Also, in the nickel-metal hydride battery, wherein the core plate exposure surface 62b is formed by removing the active material at the side disposed at the outermost side of the electrode group as described above, because the core plate 62a, which becomes the exposed surface, functions as a metal cover covering the electrode group 60a, the electrode group 60a can be easily inserted in the battery casing 65 and falling off of the active material can be effectively prevented. This is because the core plate 62a functions as the metal cover is an electrically conductive plate material such as a punching metal, and the like, in the casing of inserting the electrode group 60a into the battery casing 65, it does not occur that the active material of the outermost side of the electrode group 60a falls off by being peeled off. Furthermore, because the inside surface of the battery casing 65 can be directly electrically contacted with the core plate 62a, the high-rate discharge characteristics can be improved.
Now, for producing the electrode having formed a core plate exposed surface, there are two kinds of methods, that is, a method, wherein at coating the active material slurry onto the core plate, the active material slurry is not coated on the side of the core plate, which becomes the core plate exposed surface, and a method, wherein after coating the active material slurry onto both surfaces of the core plate, the active material coated on the side becoming the core plate exposed surface is removed.
In the casing of forming a portion of coated the active material slurry on both surfaces of the core plate and a portion of coated the active material slurry on one surface only on one sheet of a core plate, when a method of previously not coating the active material slurry on the portion of becoming the core plate exposed surface is employed, at rolling, the situation that the both surface-coated portion (the thickness of the whole body is thick) is rolled but the one-surface-coated portion (the thickness of the whole body is thin) is not rolled occurs and the problem that the productivity is lowered occurs. On the other hand, in the method, wherein after coating the active material slurry on both surfaces of the core plate, the active material coated at the side becoming the core plate exposed surface is removed, since the thickness of the whole body is uniform, the whole body of the core plate is uniformly rolled and the problem as described above does not occur.
However, in the method that after coating the active material slurry on both surfaces of the core plate, the active material coated on the side becoming the core plate exposed surface, for forming the core plate exposed surface after drying and rolling, by a means of applying a blade to the active material layer, the active material layer of one side of the core plate coated with the active material is removed by scraping. Accordingly, the work of scraping off the hardened active material layer after drying is troublesome, and a problem that the adhesion of the active material layer at the opposite side to the side of scraping off the active material layer becomes insufficient, and the quality is lowered caused by falling off of the active material occurs.
In the nickel-metal hydride battery, a positive electrode using nickel hydroxide as an active material and a negative electrode using a hydrogen absorbing alloy may be rolled via a separator to form a spiral-form electrode group.
The negative electrode is formed by coating a hydrogen absorbing alloy slurry prepared by kneading a hydrogen absorbing alloy powder, a water-soluble binder, and pure water or water on both surfaces of an electrically conductive core plate made of a punching metal holding a hydrogen absorbing alloy layer, and usually is produced, after coating the hydrogen absorbing alloy slurry on both surfaces of the electrically conductive core plate, through a step of naturally drying at room temperature (about 25xc2x0 C.). In this casing, there is a problem that when the electrode coated with the hydrogen absorbing alloy slurry is natural-dried, the drying speed is slow and usually until the electrode is dried, a long drying time of from about 5 to 6 hours is required, whereby the productivity of the electrode is lower.
Thus, for solving such a problem, a method of drying at a high temperature (about 60xc2x0 C. or higher) after coating the hydrogen absorbing alloy slurry on both surfaces of the electrically conductive core plate has been proposed. When the electrode coated with the hydrogen absorbing alloy slurry is dried at a high temperature, the electrode can be dried for a drying time of from about 15 to 30 minutes, and the productivity of the electrode is improved. Thereafter, the negative electrode thus prepared and a positive electrode are rolled in a spiral form via a separator to form a spiral-form electrode group, and by inserting the spiral-form electrode group in a metal-made outer casing together with an alkaline electrolyte and tightly sealing the metal-made outer casing, a nickel-metal hydride battery is obtained.
To increase the drying speed of the electrode for improving the productivity of the electrode, it becomes necessary to dry the electrode at a high temperature as described above. However, when the electrode is dried at a high temperature, the evaporation speed becomes fast, whereby the water contained in the hydrogen absorbing alloy layer quickly moves to the electrode surface (drying surface side) from the inside of the electrode. Thereby, because the binder contained in the hydrogen absorbing alloy layer moves with the movement of the water, the binder is omnipresent at the surface of the electrode and is solidified. As the result thereof, there occurs a phenomenon that the amount of the binder in the hydrogen absorbing alloy layer in the vicinity of the electrically conductive core plate disposed at the center portion of the electrode is reduced.
There occurs a problem that because when the amount of the binder in the active material layer in the vicinity of the electrically conductive is reduced, the adhesive strength of the conductive core plate and the hydrogen absorbing alloy is lowered, in the casing of rolling in a spiral form and in the casing of inserting the spiral-form electrode group in a metal-made outer casing, the hydrogen absorbing alloy layer is liable to fall off from the conductive core plate. Particularly, in a battery that the peripheral portion of the negative electrode disposed at the outermost periphery of the spiral electrode group is not covered by a separator to expose the negative electrode, and the exposed negative electrode is directly contacted with a metal-made outer casing for obtained a battery of a high capacity by effectively utilizing the space in the metal-made outer casing, there occurs a problem that in the casing of inserting the spiral-form electrode group into the metal-made outer casing, the hydrogen absorbing alloy layer becomes more liable to fall off from the conductive core plate.
Further, for the nickel-metal hydride battery of this kind, it has been demanded to have a higher capacity and a higher output, and it has become indispensable to insure the high energy densities of the positive and negative electrodes. To insure the high energy densities of these positive and negative electrodes, it is necessary to increase the active material packing density as high as possible, and particularly, it is desirable that in the negative electrode, the packing density is at least 4.85 g/cm3. However, in the casing of preparing a negative electrode having such a high packing density, when a hydrogen absorbing alloy powder having a large mean particle size is used, there occur problems that waviness occurs in the negative electrode obtained and creases occur in the negative electrode to lower the quality of the negative electrode obtained.
This is considered to be as follows. That is, because the negative electrode of a high packing density is produced after coating an active material sullry using the hydrogen absorbing alloy powder having a large mean particle size onto an electrically conductive core plate, when the negative electrode is pressed by applying a definite pressing force, the hydrogen absorbing alloy powder itself is not compressed and the volume of other than the hydrogen absorbing alloy powder is large, whereby the pressing force is unevenly distributed.
Thus, for solving such a problem, as the result of making various investigations on the method of preparing a negative electrode of a high packing density using a hydrogen absorbing alloy powder having a small mean particle size, it has been found that by using the hydrogen absorbing alloy powder having a mean particle size of not larger than 60 xcexcm. the occurrences of waviness and creases in the negative electrode obtained can be restrained.
However, when the hydrogen absorbing alloy powder having a mean particle size of not larger than 60 xcexcm is packed at a high density, the occurrence of waviness and creases in the negative electrode obtained can be restrained but, on the other hand, there occurs a problem that the adhesive strength of the negative electrode is lowered and the hydrogen absorbing alloy powder is fallen off from the negative electrode. This is considered to be caused by that with small sizing the mean particle size of the hydrogen absorbing alloy powder, the particle number of the hydrogen absorbing alloy powder is increased, and also the pressing force onto the negative electrode is increased by high-density packing, whereby the binder adhering the particles each other of the hydrogen absorbing alloy powder and adhering the hydrogen absorbing alloy powder and the electrically conductive core plate is crazed to cause discrepancy among the particles each other of the hydrogen absorbing alloy powder and between the hydrogen absorbing alloy powder and the conductive core plate.
Now, when a discrepancy (crazing of the binder) occurs among the particles each other of the hydrogen absorbing alloy powder or between the hydrogen absorbing alloy powder and the conductive core plate, there occurs a problem that in the casing of forming an electrode group by alternately laminating such negative electrodes and positive electrodes via separators, and in the casing of inserting the electrode group into a metal-made outer casing, the hydrogen absorbing alloy powder becomes liable to be fallen off from the negative electrodes. Particularly, in the battery that for making a battery of a high capacity by effectively utilizing the space in the metal-made outer casing, the peripheral portion of the negative electrode disposed at the outermost side of the electrode group is not covered by a separator to expose the negative electrode, and the exposed negative electrode is directly in contact with the metal-made outer casing, in the casing of inserting the electrode group into the metal-made outer casing, the hydrogen absorbing alloy powder becomes more liable to be fallen off from the negative electrode.
The present invention has been made for solving the above-described problems, and an object of the invention is to provide a method of producing a high-quality alkaline storage battery by increasing the adhesive strength of the active material layer of the opposite side to the side of scraping off the active material layer, whereby falling off the active material can be prevented.
Another object of the invention is to provide a production method capable of restraining lowering of the adhesive strength if a negative electrode even by improving the productivity by increasing the drying temperature of a negative electrode coated with a hydrogen absorbing alloy, whereby the hydrogen absorbing alloy layer can be prevented from falling off from an electrically conductive core plate, and an alkaline storage battery of a high quality is obtained.
Yet another object of the invention is to provide a method of producing an alkaline storage battery, which can restrain the occurrence of lowering the adhesive strength of negative electrodes even by using the hydrogen absorbing alloy powder having a small mean particle size for increasing the packing density of the active material, whereby the hydrogen absorbing alloy powder can be prevented from falling off from the negative electrode and obtain an alkaline storage battery of a good quality having a long life.
According to first aspect of the invention, a method of producing an alkaline storage battery includes a coating step of coating an active material slurry comprising an active material, a binder, and a solvent for the binder on both surfaces of the core plate, a drying step of drying the electrode coated with the active material slurry, an active material removing step of removing the active material of the side of forming the exposed surface of the core plate, and a solvent-attaching step of attaching the solvent for the binder from the exposed surface side of the core plate.
According to second aspect of the invention, a method of producing an alkaline storage battery includes a coating step of coating a hydrogen absorbing alloy slurry comprising the above-described hydrogen absorbing alloy powder, a binder, and a solvent for the binder onto both surfaces of an electrically conductive core plate to form a coated electrode, a drying step of drying the coated electrode to form a dry electrode, a solvent-attaching step of attaching the above-described solvent for the binder to the surface of the dry electrode, and a low-temperature drying step of drying the dry electrode at a temperature lower than the drying temperature in the above-described drying step.
According to third aspect of the invention, a hydrogen absorbing alloy electrode comprises an electrically conductive core plate having attached thereto a mixture containing at least a hydrogen absorbing alloy powder and a binder capable of re-dissolving, in that the mean particle size of the hydrogen absorbing alloy powder is not larger than 60 xcexcm, and the packing density of the hydrogen absorbing alloy is at least 4.85 g/cm2, and the hydrogen absorbing alloy powders each other and the hydrogen absorbing alloy powder and the electrically conductive core plate are adhered by the above-described binder capable of re-dissolving.